iCloud Backups

Recently I removed the SIM from my 4S and after some time I realized that the Viber messages along with some other content was gone. Someone told me that this was a security mechanism. I tried to restore the missing content by completely resetting my iPhone and restoring from an earlier iCloud backup. Now, here comes the weird part, the backups listed were indeed from my device but the Apple ID that the phone asked me to confirm in order to proceed with the media restore was not mine. Even after the restore, the phone insisted with the confirmation of that particular account. I logged in with my Apple ID and then I got a message telling me that the process was unsuccessful. Does this mean that iCloud was compromised and someone got access to my account? What’s the implication of this? Are iCloud backups encrypted? Do I have to change every single password and authenticator access that was on my phone? Anyone encountered the same issue?